The present invention relates to methods for fabricating semiconductor devices, and more particularly relates to semiconductor device fabrication methods which enable semiconductor light emitters, such as semiconductor laser devices, to be mounted in a self-aligned manner.
Typical digital-versatile-disc (hereinafter referred to as xe2x80x9cDVDxe2x80x9d) players need to function to play back compact discs (hereinafter referred to as xe2x80x9cCDsxe2x80x9d) in addition to DVDs, and also have to function to replay, and store data on, recordable CDs (CD-Rs) which have become widespread rapidly in recent years.
As a laser for replaying DVDs, a red laser with a wavelength in the 650 nm band is employed, while an infrared laser with a wavelength in the 780 nm band is used as a laser for playing back CDs and CD-R discs. In the currently available DVD players, therefore, two semiconductor laser diodes are incorporated in the form of an array: one is a red semiconductor laser diode for generating a red laser beam and the other is an infrared semiconductor laser diode for generating an infrared laser beam.
With an increasing demand for smaller personal computers and other information equipment, DVD players also need to be reduced further in size and thickness. To that end, it is indispensable to reduce the size and thickness of optical pickup. Methods for reducing optical pickup in size and thickness include optical system simplification.
As a method for simplifying an optical system, integration of a red semiconductor laser diode and an infrared semiconductor laser diode is available. The current DVD players include two optical systems: one for a red semiconductor laser diode and the other for an infrared semiconductor laser diode. Integration of the red semiconductor laser diode and the infrared semiconductor laser diode allows one optical system to be shared, thereby realizing an optical pickup system of smaller size and thickness.
For instance, as one example of the integration of a red semiconductor laser diode and an infrared semiconductor laser diode, a so-called monolithic semiconductor laser diode array which is integrated on a substrate is disclosed in Japanese Laid-Open Publication No. 11-186651.
Japanese Laid-Open Publication Nos. 11-144307 and 11-149652 disclose another example, in which hybrid integration of two semiconductor laser chips, one for a red laser and the other for an infrared laser, enables an optical system to be shared in an optical pickup system.
Nevertheless, in the conventional monolithic two-wavelength laser diode array, the respective active layers of the laser diodes have different compositions and thus have to be grown in different process steps, which results in the problem of low yields. In particular, when high-output laser diodes are monolithically integrated, yields decrease significantly.
Moreover, it is very difficult, in the viewpoint of crystal growth, to monolithically integrate a gallium nitride (GaN)-based blue laser diode, which is used in high density DVDs, and an aluminum gallium indium phosphide (AlGaInP)-based red laser diode, which is used in typical (conventional) DVDs.
The conventional hybrid optical pickup, on the other hand, have the problem that when the red semiconductor laser chip and the infrared semiconductor laser chip are assembled using assembly equipment, it is difficult to adjust and optimize the locations of the active layers of the semiconductor laser chips and the distance between the light emitting points thereof.
In recent years, mounting methods in which a fluidic self-assembly (hereinafter referred to as xe2x80x9cFSAxe2x80x9d) technique is used have been developed as one type of device-mounting method.
In the FSA technology, devices (hereinafter referred to as xe2x80x9cfunction blocksxe2x80x9d) ranging in size from 10 xcexcm to several hundred xcexcm and having given shapes are suspended into a liquid to form a slurry. The liquid (suspension) in the form of slurry is poured over the surface of a substrate of, e.g., silicon having recessed portions therein. The recessed portions are substantially the same as the function blocks in size and shape. In this manner, the function blocks that have been dispersed in the liquid are engaged into the recessed portions and thereby mounted onto the substrate.
The FSA technology is disclosed in U.S. Pat. Nos. 5,545,291, 5,783,856, 5,824,186 and U.S. Pat. No. 5,904,545, for example.
However, the conventional FSA process has the problem that the formation, by etching, of the recess structure used for the engagement of the function blocks with the substrate is not easy and that the productivity of the mounting substrate is thus low.
An object of the present invention is therefore that in the fabrication of a semiconductor device formed by hybrid integration of semiconductor chips, the semiconductor chips can be easily and reliably mounted while using the FSA technology.
In order to achieve the object, in inventive semiconductor device fabrication methods, instead of forming in a substrate itself a recess structure into which semiconductor chips are disposed, the semiconductor chips are disposed into openings formed in a template which correspond to a layout pattern for the semiconductor chips.
Specifically, an inventive semiconductor device fabrication method includes the steps of: (a) forming a template having openings that are located to correspond to a pattern in which a plurality of semiconductor elements in the form of chips are to be arranged, (b) holding the template on the principal surface of a substrate on which the semiconductor elements are to be arranged, and (c) spreading the semiconductor elements into a liquid and pouring the semiconductor-element-spread liquid over the substrate on which the template is held, thereby allowing the semiconductor elements to be disposed into the respective openings in the template in a self-aligned manner.
According to the inventive semiconductor device fabrication method, a recess structure, into which the semiconductor elements are disposed, does not have to be formed directly in the principal surface of the substrate on which the semiconductor elements are to be disposed. As described above, since the inventive method employs the template having the openings into which the semiconductor elements are engaged, a recess structure, which is difficult to form, does not have to be formed in the substrate, such that the semiconductor device in which the semiconductor elements are hybridly integrated can be easily and reliably fabricated.
In the inventive semiconductor device fabrication method, in the step (c), it is preferable that the semiconductor-elements dispersed liquid is poured with the template-holding substrate being rotated in its principal surface.
Then, the semiconductor elements are more likely to be settled into the openings of the template, which increases throughput in the mounting process.
The inventive method preferably further includes, before the step (b), the step (d) of forming a plurality of substrate electrodes for making the associated semiconductor elements electrically conductive, on parts of the principal surface of the substrate where the semiconductor elements are to be arranged.
Alternatively, the inventive method preferably further includes, between the steps (b) and (c), the step (d) of forming a plurality of substrate electrodes for making the associated semiconductor elements electrically conductive, on parts of the principal surface of the substrate that are exposed through the openings of the template.
In those cases, the inventive method preferably further includes, after the step (c), the step (e) of securing the semiconductor elements to the associated substrate electrodes, and then removing the template from the substrate. Then, the template, once it is made, can be reused.
In the inventive method, the semiconductor elements are preferably edge-emitting semiconductor laser elements that emit a laser beam from a facet, and in the step (a), the openings of the template are preferably formed so that the respective emission directions of the semiconductor laser elements are aligned in one direction.
Then, only by the disposition of the semiconductor laser elements into the openings of the template, the locations of the respective active layers of the semiconductor laser elements self-align with each other, and the respective light-emitting points of the semiconductor laser elements also self-align with each other at uniform distances. In addition, the semiconductor laser elements are capable of being integrated, even if their constituents (compositions) differ from each other. Further, the openings themselves function to align the emission directions of the semiconductor laser elements, such that wavefront aberration of a spot formed when the resultant semiconductor device is used in an optical pickup system can be reliably within an allowable range.
In this case, in each of the semiconductor laser elements, optical output from a front facet is preferably equivalent in value to optical output from a rear facet.
Then, it is not necessary to select the emission direction in the semiconductor laser elements, which further facilitates the manufacture of the semiconductor device.
In the inventive method, in the step (a), the openings of the template are preferably formed to correspond to the configurations of the semiconductor elements on the side thereof on which the semiconductor elements are disposed.
In that case, the semiconductor elements preferably differ from each other in configuration on the side thereof on which the semiconductor elements are disposed into the associated openings. Then, the semiconductor elements can be selectively disposed into the associated openings of the template.
Further, in that case, the semiconductor elements are preferably semiconductor light emitters, and in the step (a), the openings of the template preferably differ from each other in configuration in accordance with the emission wavelengths of the semiconductor light emitters. Then, if the light emitters are laser emitters, two-wavelength laser chip arrays can be obtained.
Alternatively, in that case, the semiconductor elements are preferably semiconductor light emitters, and in the step (a), the openings of the template preferably differ from each other in configuration in accordance with the optical output values of the semiconductor light emitters. Then, if the light emitters are laser diodes, laser chip arrays in each of which different functions, such as writing and reading, can be performed are obtainable.